rust: auxiliary: implement Sync for Device<Bound>

rust: auxiliary: implement Sync for Device<Bound>

Implement Sync for Device<Bound> in addition to Device<Normal>.

Device<Bound> uses the same underlying struct auxiliary_device as
Device<Normal>; Bound is a zero-sized type-state marker that does not
affect thread safety.

This is needed for drivers to store &'bound auxiliary::Device<Bound> in
their private data while remaining Send.

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Uwe Kleine-König <ukleinek@kernel.org>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Link: https://patch.msgid.link/20260525202921.124698-9-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>